Electronic fuel control systems are increasingly being used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems vary the amount of fuel delivered for combustion in response to multiple system inputs including throttle angle and the concentration of oxygen in the exhaust gas produced by combustion of air and fuel.
Electronic fuel control systems operate primarily to maintain the ratio of air and fuel at or near stoichiometry. Electronic fuel control systems operate in a variety of modes depending on engine conditions, such as starting, rapid acceleration, sudden deceleration, and idle. One mode of operation is known as closed-loop control. Under closed-loop control, the amount of fuel delivered is determined primarily by measuring the concentration of oxygen in the exhaust gas to determine the extent to which the ratio of air to fuel (A/F) in the ignited mixture deviates from stoichiometry.
The oxygen in the exhaust gas is typically sensed by a heated exhaust gas oxygen (HEGO) sensor. The electronic fuel control system adjusts the amount of fuel being delivered in response to the output of the HEGO sensor when the sensor output indicates a rich air/fuel ratio, below stoichiometry the control system decreases the amount of fuel delivered while the detection of a lean air/fuel ratio increases the fuel flow.
The effective operation of closed-loop fuel control systems using exhaust gas sensors is complicated by the physical transport delay experienced by a given mass of fuel and air as it travels from the intake manifold through the engine and exhaust system to the HEGO sensor. This transport delay prevents the system from promptly detecting and responding to undesirable air/fuel ratios, resulting in reduced catalyst conversion efficiencies and an increase in HC, CO and NOx emissions.